It is known that a converter for a satellite receiver, commonly referred to as an "LNB," is supplied by a device that also performs logic control of the LNB.
In particular, such a device, sometimes referred to as an "LNBP," is a dual-output voltage supply having a regulated output voltage that can be amplitude-modulated by means of a trapezoidal signal generated in the device itself. The modulating signal is detected by the satellite receiver for band control.
The modulating signal should have characteristics, in terms of frequency, duty cycle, rise and fall times, which comply with standards used by many satellite receiver manufacturers.
A difficult problem to solve is controlling rise and fall times of the modulating signal, which have an upper limit and a lower limit. The lower limit is required because very fast wavefronts, generated by high-frequency harmonics, can compromise satellite signal reception quality.
The upper limit is instead necessary because very slow wavefronts are comparable with the period of the modulating signal and would, therefore, compromise recognition of the modulating signal by the satellite receiver.
The problem is currently addressed by using discrete components, such as by using a square-wave oscillator followed by a low-pass RC filter. This device powers and controls the satellite receiver by means of a coaxial cable. Control of a reception band of the satellite receiver is performed by means of a square-wave signal with controlled fronts.
FIG. 1 illustrates a square-wave oscillator 1 followed by a low-pass pass RC filter 2. In this manner, a square-wave signal from the oscillator is filtered by the RC filter 2, whose output signal is a trapezoidal waveform signal having an amplitude equal to an amplitude of the square-wave signal generated by the oscillator 1.
The trapezoidal output signal is added to a reference voltage so that the output of the device is amplitude-modulated with a preset peak-to-peak amplitude of the modulating signal. The sum of the trapezoidal waveform signal and the reference voltage is provided by the OR gate 3. The output of the OR gate 3 is sent to an operational amplifier 20 having a transistor 21 connected to an output thereto.
This technique has inaccuracies and has a drawback of having little correlation between the frequency of the modulating signal and the rise and fall times of the modulating signal. In particular, there is no link between the frequency of the square wave and the rise and fall times. In this manner, it is possible to have a high frequency and long-lasting fronts, thereby leading to problems in recognizing the signal.
Accordingly, this system requires dual trimming of the capacitances of the oscillator 1 and of the RC filter 2. Furthermore, the above-described system provides low precision in the amplitude of the modulating signal, and further entails a need to normalize the amplitude of the modulating signal with respect to the reference voltage, such that the amplitude of the modulated output is preset.